For increased transfer efficiency, paints and other atomized or particulate coatings are often electrostatically charged when applied to a workpiece. A very high voltage electrostatic charge is imparted to the atomized paint droplets or other coating particles relative to the workpiece. The charge causes the droplets to be attracted to the workpiece, even when they are initially moving on a path away from the workpiece. Solvent based coatings generally are easily charged because they are electrically nonconductive. Typically, a high voltage electrode is located at the spray gun nozzle for contacting the paint as it is discharged and atomized. However, precautions must be taken to prevent sparking from the electrode since solvent based paints generally are flammable. Direct charging is unsatisfactory when an electrically conductive paint, such as a water borne paint, or a semi-conducting paint is being applied. Normally, the paint is supplied to the gun from an electrically grounded remote source. The high voltage on an electrode exposed to the paint column at the gun nozzle will be dissipated through the paint column to the grounded paint source. Consequently, the paint is not adequately charged.
Various techniques have been used in the past for electrostatically applying electrically conductive and semi-conductive paints and other coating materials. One technique is to electrically isolate the entire paint supply from ground. During coating, the paint from the atomizer to the source is maintained at a high voltage. Extra care must be taken with a system of this type in protecting the operator from contact with the high voltage and in preventing arcing from all portions of the paint supply. Further, this type of system provides a greater risk because of the large charged capacitance formed by the paint supply system. As the stored high voltage energy increases, there is a greater risk of harm from any spark discharge. According to another technique, paint is supplied from a grounded source to an intermediate reservoir which is isolated from ground. Various arrangements have been used to provide a voltage block between the grounded source and the isolated reservoir which is maintained at a high voltage at lease during spraying. Still another technique involves indirectly charging the paint after it is atomized by establishing a strong electrostatic field adjacent the paint discharge end of the atomizer. The atomized paint becomes charged as it passes through the field. This technique permits grounding the atomizer and the entire paint supply system.
In one type of indirect charging apparatus, an electrically insulated annular or hoop shaped element is supported from a rotary atomizer to be spaced outwardly from and slightly behind the paint discharge edge. A plurality of small needle like electrodes are uniformly spaced around the element and are positioned to point in a forward direction toward the workpiece being coated. An electrical conductor embedded in the element connects the electrodes together to a high voltage source. In order to prevent conductive deposits from accumulating on the element which in turn will reduce the strength of the electrostatic field produced by the electrodes, the portions of the electrodes projecting from the element are embedded in insulation. The insulation on the electrodes also is required for safety. Even though a large value resistor may be placed in the power supply lead to limit the available current and voltage as a grounded object approaches an electrode, the resistor has limited effect because it is located before the metal conductor in the annular element. The metal conductor which interconnects the electrodes has a distributed capacitance which becomes charged with the high voltage. There is no guarantee that water borne or other conductive and semi-conductive paints do not contain flammable or incentive solvents or that the user will not on occasion switch to flammable solvent based paints. Sufficient energy may be stored in the capacitance formed by the metallic conductor to ignite any flammable solvents if a grounded object is brought sufficiently close to an electrode to cause a spark.